D&D: so Bag of Holding.
The game does mention she is kitted out Boots of Striding and Springing so she like has other magic bits and bobs.

So something's just occurred to me.

How does she actually carry away the things she steals?

She doesn't have a sack and the bags she has are far too small to carry anything, as they presumably contain her lockpicks and other tools.

I can't imagine the whole lightweight, quiet getup does much good if you can't even make away with even a silver fork once you get in.